The present invention relates to an apparatus for measuring stress at the surface of a polycrystalline structure by means of X-ray diffraction techniques. It is well known, that when an X-ray beam strikes a crystal, part of the incident energy is transmitted, part thereof is absorbed by the crystal and part is diffused or diffracted, that is to say that radiation of the same wavelength as the incident radiation is emitted in directions differing from those of the incident primary beam.
The diffraction conditions are determined by Bragg's law .lambda. = 2 d sin .theta. which is a simple expression of the conditions of phase matching between the waves diffracted by the different atoms of the crystal. In this formula, .lambda. is the wavelength, d is the interplanar spacing i.e. spacing between the neighboring planes of a set of parallel lattice planes, and .theta. is the diffraction angle i.e. the angle of the incident (or diffracted) beam in relation to the set of parallel lattice planes.
Thus, it will be seen, that, as will be familiar to those skilled in the art, the measurement of the diffraction angle makes it possible, through use of the Bragg relationship, to determine in the interplanar spacing if the wavelength of the X-ray radiation is known.
The existence of a stress at the surface of a polycrystalline body produces a modification of the shape of the crystal lattice, which can be measured as a variation .DELTA. d of the interlattice spacing d of the set of lattice planes.
The determination of stress can thus be carried out by measuring the value .DELTA. d/d.
Excellent prior art equipment for laboratory use is available in the shape of the counter-type diffractometer.